||Neuregulins (NRGs) are a family of growth factors, among which are nerve-derived signaling molecules identified to be important for the formation of the postsynaptic specialization in the neuromuscular junction (NMJ). Type I isoforms of NRGs are synthesized as transmembrane proteins (TM-NRGs). The cytoplasmic tail of the TM-NRGs display more than 85% amino acids sequence identity in rat, human and chicken, suggesting that this long intracellular domain has important biological functions. It is tempting to speculate that the various regions of TM-NRGs may play different functional or regulatory roles in NRG signaling probably through physical interaction with different proteins. Therefore, the objective of this study is to screen for proteins interacting with the cytoplasmic tail of TM-NRGs by yeast two hybrid system. By using the full length rat TM-NRG a-tail variant as bait, a total of 4 positive clones were obtained in the screening of the adult mouse brain cDNA library. One of them was found to be the neuronal specific cytoskeletal protein, the neurofilament middle polypeptide (NFM). Yeast two hybrid studies also showed that the interaction was mediated by the common region of the TM-NRG cytoplasmic tail and the carboxyl tail region of NFM. Immunostaining of rat primary granule cell culture showed that they were co-localized at the axon hillock and along the axon of the granule cells. Western blot analysis of mouse brain lysates showed that NFM expression was increased upon embryonic day 20 whereas NRG expression remained high throughout development. These results suggested that they may potentially interact with each other in vivo. The two proteins were also expressed in transfected COS-7 cells, which would help facilitate further studies of their interaction in mammalian cells. Taken together, the present study had identified NFM as a strong candidate of protein interacting with the cytoplasmic tail of TM-NRGs.